Using More Than One Audio Device

Ardour will only ever deal with a single audio device. If you
want to use more than one, you have two choices:

If you want to use Ardour to start JACK (which handles all
audio I/O) you will need to create a "fake" audio device on your
computer the represents all the multiple devices you wish to
use. How to do this is platform dependent and described below.

Use a different tool to start JACK and manage all the devices.

Ardour is fundamentally designed to be a component in a
pro-audio/music creation environment. Standard operating practice
for such setups involves using only a single digital sample
clock (something counting off the time between audio samples).
This means that trying to use multiple independent soundcards is
problematic, because each soundcard has its own sample clock, running
independently from the others. Over time, these different clocks
drift
out of sync with each other, which causes glitches in the audio. You
cannot stop this drift, although in some cases the effects may be
insignificant enough that some people might not care about them.

Thus in an ideal world you should not use multiple independent
soundcards but instead use a single device with a single clock and all
the inputs, outputs and other features that you need.

Of course, a lot of people don't live in an ideal world, and believe
that software should make up for this.

OS X

In CoreAudio, aggregate devices provide a method to use
multiple soundcards as a single device. For example, you can
aggregate two 8-channel devices so that you can record 16 channels
into Ardour.

If you are using a single typical 3rd party
audio interface (such as those from Apogee, RME, Presonus, and many
others), or you are using JackPilot or a similar
application to start JACK, you do not need to worry about this.
You will need to set up an aggregate device only if either
of the following conditions are true:

You want to use two entirely separate
devices and want to start JACK using Ardour.

You want to use your builtin audio deviceand
want to start JACK using Ardour.

You want to use more than two entirely separate devices

In the case of your builtin audio device, you will need to create
an aggregate device that combines "Builtin Input" and "Builtin
Output" into one device.

The precise instructions for creating an aggregate device on OS X
have varied from one released to another.